Polymer nanocomposites-major conclusions and achievements reached so far

The dielectric properties of a number of polymeric nanocomposites (PNC) have been investigated and reported, and there are very good reviews available, for example, see [1]-[3]. CIGRE Working group D1.24 has also performed several collaborative investigations on mostly epoxy- and polyethylene-based nanocomposites, which are reported in CIGRE publications [4], [5] as well as in archived papers [6], [7]. Dielectric nanocomposites investigated in the literature include various polyolefins such as polyethylene (PE; and PE blends) and polypropylene, ethylene vinyl acetate, polyamine, epoxy, and elastomers such as silicone rubber, containing various nanofillers such as metallic oxides, silica, alumina, titanium oxide, zinc oxide, and layered silicates (clays). Due to the very high specific surface area of nano-sized fillers, a few percent addition can significantly affect the dielectric properties of a polymeric material. The most common and practical processing methods suitable for thermoplastic nanocomposites are melt compounding, using a mixer, extruder, or both, and mixing in the liquid phase prior to polymerization for thermosetting resins, a process commonly called the in situ polymerization process [8]. Figure 1 gives examples of typical microstructures of polyolefin-based nanocomposites processed by melt compounding. The striking similarity of the microstructure shown in Figures 1(b) and 1(c) should be noted as both were obtained in two different labs from the melt compounding of fumed silica and a thermoplastic resin using a twin screw extruder. Similar microstructures are also reported for isotactic polypropylene/SiO₂ nanocomposites melt blended by extrusion [9].